The present invention relates to a honeycomb structure used in a filter for purification of automobile exhaust gas, a catalyst carrier, or the like, as well as to a process for production of such a honeycomb structure.
Porous honeycomb structures are in wide use as a filter for capturing and removing the particulate substance present in a dust-containing fluid (e.g. exhaust gas emitted from diesel engine), or as a catalyst carrier for loading thereon a catalyst component capable of purifying the harmful substances present in an exhaust gas. It is known that as a material constituting such a honeycomb structure, there are used refractory particles such as silicon carbide (SiC) particles and the like.
As a specific technique related thereto, there is disclosed, in, for example, JP-A-6-182228, a porous, silicon carbide-based catalyst carrier of honeycomb structure, obtained by using, as a starting material, a silicon carbide powder having a given specific surface area and a given impurity content, molding the material into a desired shape, drying the molded material, and firing the resulting material at a temperature of 1,600 to 2,200xc2x0 C.
Meanwhile, there are disclosed, in JP-A-61-26550, a process for producing a vitrifying material-containing refractory product, which comprises adding a vitrifying material to an easily oxidizable material or a refractory composition containing an easily oxidizable material, mixing, kneading and molding them together with a binder, and open-firing the molded material in a furnace containing a non-oxidative atmosphere; and, in JP-A-8-165171, a silicon carbide molded material obtained by adding, to a silicon carbide powder, an organic binder and inorganic binders of clay mineral series, glass series and lithium silicate series and molding the resulting material.
Also, in JP-A-6-182228 is introduced a process for producing a conventional porous, silicon carbide-based sintered body, which comprises adding, to silicon carbide particles as an aggregate, a binder such as vitreous flux, clayey material or the like, molding them, and firing the molded material at a temperature at which the binder melts.
Further, as to a high-temperature use ceramic filter produced by molding refractory particles which consist of silica sand, a ground pottery, a metal oxide (e.g. Al2O3, TiO2, or ZrO2), silicon carbide, nitride, boride, other refractory material, or the like and which are adjusted to a given grain size, to a porous, bottomed cylindrical material using a refractory binder such as water glass, frit, glaze or the like, there are disclosed, in JP-B-61-13845 and JP-B-61-13846, the preferred average particle diameter and particle size distribution of refractory particles, the preferred porosity, average pore diameter, pore volume and partition wall thickness of cylindrical material, etc.
In the sintering (necking between particles) caused by the recrystallization of silicon carbide powder per se, shown in JP-A-6-182228, the silicon carbide component vaporizes from the surfaces of silicon carbide particles and the vaporized silicon carbide component condenses at the contact areas (necks) between silicon carbide particles; as a result, the necks grow and the particles are bonded to each other. There are problems, however, that this method brings a high cost since a very high firing temperature is required to be employed in order to vaporize silicon carbide, and that the yield after firing is reduced since a material of high thermal expansion coefficient is required to be fired at a high temperature as well.
The above process allows production of a porous body of high strength; however, the porous body has a high Young""s modulus which is derived from the physical properties possessed by the silicon carbide used as a raw material.
In general, coefficient (R) of thermal shock resistance is shown by the following formula (1). In the following formula, S is a fracture strength, xcexd is a Poisson ration, E is a Young""s modulus, and xcex1 is a thermal expansion coefficient. xcexd and xcex1 are characteristic values of each material and are almost constant in each material; meanwhile, S and E vary greatly depending upon the porosity, fine structure, etc. of each material.
R=S(1xe2x88x92xcexd)/Excex1xe2x80x83xe2x80x83(1)
As shown in the above formula, thermal shock resistance is directly proportional to strength but inversely proportional to Young""s modulus. Therefore, according to the process for producing a sintered body disclosed in JP-A-6-182228, a sintered body having a sufficient thermal shock resistance, while it has a high strength though, can not be produced since Young""s modulus becomes high.
Meanwhile, there is a problem that a localized heat generation is caused by the low thermal conductivity of the filter in the case of the technique of bonding a silicon carbide powder (as a raw material) with a vitreous material, shown in JP-A-61-26550 and JP-A-6-182228 wherein a low firing temperature of 1,000 to 1,400xc2x0 C. is employed; if one tries to burn the particulates collected by and deposited on the filter for reactivation of the filter, in the case that the sintered body produced by the technique is used, for example, as a diesel particulate filter (DPF) for removing the particulates contained in the exhaust gas emitted from a diesel engine.
Further, the filter shown in JP-B-61-13845 and JP-B-61-13846 is porous but is a bottomed cylindrical material having a large partition wall thickness of 5 to 20 mm; therefore, there is a problem that the filter is not usable under the high space velocity (SV) condition like a filter for purification of automobile exhaust gas.
The present invention has been made in view of the above-mentioned situation of the prior art, and is intended to provide a honeycomb structure which contains refractory particles such as silicon carbide particles and the like and yet can be produced at a relatively low firing temperature at a low cost, which has a high strength and a high thermal shock resistance, and which can be suitably used, for example, as a filter for purification of automobile exhaust gas by a treatment such as clogging of through-channel at its inlet or outlet, or as a catalyst carrier, even under a high SV condition; and a process for producing such a honeycomb structure.
According to the present invention there is provided a honeycomb structure made of a silicon carbide-based porous body and having a plural number of through-channels extending in the axial direction, separated by partition walls, characterized in that the strength and Young""s modulus of the silicon carbide-based porous body satisfy the following relation:
Strength (MPa)/Young""s modulus (GPa)xe2x89xa71.1.
In the present invention, the strength and Young""s modulus of the silicon carbide-based porous body preferably satisfy the following relation:
Strength (MPa)/Young""s modulus (GPa)xe2x89xa71.25.
Further in the present invention, the strength and Young""s modulus of the silicon carbide-based porous body preferably satisfy the following relation. In the present invention, the silicon carbide-based porous body preferably contains silicon carbide particles as aggregate and metallic silicon as a binder:
Strength (MPa)/Young""s modulus (GPa)xe2x89xa71.3.
According to the present invention, there is also provided a process for producing a honeycomb structure, which comprises adding metallic silicon and an organic binder to raw material silicon carbide particles, mixing and kneading them to obtain a readily formable puddle, molding the readily formable puddle into a honeycomb-shaped molded material, calcinating the molded material to remove the organic binder in the molded material, and firing the resulting material, characterized in that the addition amount of the metallic silicon is 15 to 40% by weight based on the total amount of the raw material silicon carbide particles and the metallic silicon.
In the present invention, the firing is preferably conducted in a temperature range of 1,400 to 1,600xc2x0 C.